Ink jet printing has the inherent advantage of being a plain paper compatible, direct marking technology. "Continuous stream" and "drop on demand" ink jet print heads have been developed to exploit that advantage. Unfortunately, however, the nozzles which are used in conventional ink jet print heads are expensive to manufacture and are a significant source of maintenance problems.
Others have proposed nozzleless droplet ejectors for liquid ink printing. For example, Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 24, 1981 on a "Liquid Drop Emitter," describes a print head in which a piezoelectric transducer having a hemispherically shaped focusing lens is submerged in a reservoir of ink to generate a spherically focused ultrasonic acoustic wave for exciting the ink near the surface of the reservoir sufficiently to eject individual droplets of ink. Furthermore, a copending and commonly assigned United States patent application of C. F. Quate et al. Ser. No. 776,291 which was filed Sept. 16, 1985, now abandoned, and refiled on Jan. 5, 1987 as continuation Ser. No. 946,682 on a "Nozzleless Droplet Ejector," describes an improved droplet ejection mechanism in which one or more relatively low cost, planar piezoelectric transducers having interdigitated electrodes are provided for generating the focused ultrasonic acoustic waves which are employed in nozzleless print heads of the foregoing type.
As a general rule, liquid ink printing requires substantial control over the timing of the drop ejection process. The transducers of nozzleless print heads of the above-described type may be driven by amplitude modulated rf signals to provide the necessary timing control, but the electronics needed to modulate a rf signal are expensive. Thus, as pointed out in the aforementioned Quate et al. application, the preferred approach is to provide timing controllers which operate independently of the transducers. Under those circumstances, the transducer or transducers may be driven by a relatively inexpensive rf signal generator to excite the ink to a subthreshold, incipient energy level for droplet emission, thereby enabling the timing control or controllers to selectively destabilize the excited ink so that individual droplets are ejected on command.
Some liquid ink printing processes, such as matrix printing, are easier and less costly to implement if there also is provision for directionally steering the ink droplets. In recognition of that, some of the transducers disclosed in the above-identified Quate et al. application are configured to generate focused acoustic waves having a directionally controlled asymmetry.